Fix same-origin issues when referencing SVG symbols

[montalvomiguelo]

WHY are these changes introduced?

These changes are made to resolve same-origin issues when referencing symbols with the use tag from SVG files in themes during development.

WHAT is this pull request doing?

Adds SVGs to the local assets middleware.

How to test your changes?

• Serve a theme with an SVG asset containing icons/<symbol>s.
• Reference an icon from the SVG asset using the <use> tag and the asset_url filter.
  <svg><use href="{{ 'icons.svg' | asset_url }}#icon-login"/></svg>
• The icon should load as expected.

cc @karreiro (:

Post-release steps

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes